Allow `paradigm.get_data()` to return non-loaded Epochs

[PierreGtch]

Since PR #408, we theoretically have the possibility to return non-loaded (i.e. preload=False) epochs and raws from the cache. However, because the paradigms' get_data concatenates epochs, they are loaded anyway by mne (see https://mne.tools/stable/generated/mne.concatenate_epochs.html).

It would be nice to have the option to get non-concatenated epochs out of a paradigm. Maybe paradigm.get_data(..., concatenate=False), or another change in the API?

[PierreGtch]

• [ ] If this feature gets implemented, we should demonstrate it in the disk_cache example